The present invention relates to Mgxe2x80x94Al-based hydrotalcite-type particles, a chlorine-containing resin stabilizer, and a process for producing the Mgxe2x80x94Al-based hydrotalcite-type particles. More particularly, the present invention relates to Mgxe2x80x94Al-based hydrotalcite-type particles having a large plate surface diameter and an adequate thickness which are suitable as a stabilizer for chlorine-containing resins; the chlorine-containing resin stabilizer; and a process for producing such Mgxe2x80x94Al-based hydrotalcite-type particles.
As layered compounds, there are known various compounds as well as clay minerals or the like. Among these layered compounds, layered double hydroxides such as hydrotalcite have such a structure capable of inserting various ions or molecules into spacings between respective layers thereof and, therefore, can exhibit an anion-exchanging property.
In general, as described in Journal of the Chemical Society of Japan, 1995, No. 8, pp. 622 to 628, the hydrotalcite is represented by the formula:
xe2x80x83[M2+1-xM3+x(OH)2]x+[Anxe2x88x92x/n.yH2O]xxe2x88x92
(wherein M2+ is a divalent metal ion such as Mg2+, Co2+, Ni2+ or Zn2+; M3+ is a trivalent metal ion such as Al3+, Fe3+ or Cr3+; Anxe2x88x92 is an n-valent anion such as OHxe2x88x92, Clxe2x88x92, CO32xe2x88x92 or SO42xe2x88x92; and x is usually 0.2 to 0.33,)
and such hydrotalcite has a laminated crystal structure which comprises two-dimensional main layers composed of regular octahedral brucite units each having a positive charge, and interlayers each having a negative charge.
The hydrotalcite has been used in various applications because of good anion-exchanging property thereof, for example, as ion exchange materials, adsorbents, deodorants or the like. Also, the hydrotalcite has been used in various other applications such as stabilizers for resins or rubbers, e.g., polyethylene, polypropylene and chlorine-containing resins (typically, vinyl chloride resins), as well as paints, various catalysts, agricultural films, inks or the like.
In particular, as gasket materials the chlorine-containing resins have been widely used because the chlorine-containing resins are more excellent in flexibility and adhesion property than those of rubbers, thermoplastic elastomers or the like.
However, the chlorine-containing resins are susceptible to heat deterioration and oxidation degradation and, therefore, suffer from deteriorated properties such as low elongation when used for a long period of time, thereby causing such a problem that the resins are no longer usable as gasket materials.
In recent years, as to catalysts or the like, it has also been required to use those containing no harmful metals from the standpoint of environmental protection. The hydrotalcite-type particles have been expected to satisfy these requirements because the hydrotalcite-type particles have almost no toxicity and exhibit an excellent catalytic property or the like.
In particular, among the hydrotalcite-type particles, Mgxe2x80x94Al-based hydrotalcite-type particles containing Mg2+ as a divalent metal ion and Al3+ as a trivalent metal ion, are most noticeable owing to its good stability. There have also been known iron-based hydrotalcite-type particles (Japanese Patent Application Laid-Open (KOKAI) No. 9-227127(1997)). However, when the iron-based hydrotalcite-type particles are kneaded with resins, the obtained kneaded material is disadvantageously colored due to inclusion of the iron. Further, when used as a stabilizer for chlorine-containing resins, the iron-based hydrotalcite-type particles cannot exhibit a sufficient heat resistance as compared to the Mgxe2x80x94Al-based hydrotalcite-type particles.
As the general production method of hydrotalcite, there is known a method of mixing an aqueous metal salt solution containing divalent metal ions and trivalent metal ions which constitute main layers thereof, with an aqueous carbonate solution containing carbonate ions which constitute interlayers thereof, and then subjecting the obtained mixture to coprecipitation reaction while controlling the temperature, the pH value and the like (Eiichi NARITA, xe2x80x9cChemistry of Layered Double Hydroxides as a Fixing Agent of Anionsxe2x80x9d in xe2x80x9cSurface Techniquesxe2x80x9d, pages 722-727, 1993). In addition to the above method of conducting the reaction under ordinary pressure, there is also known a method of producing hydrotalcite under pressure by hydrothermal reaction using an autoclave.
Hitherto, hydrotalcite-type particles to be kneaded into resins, have been required to possess a large plate surface diameter and an adequate thickness in the consideration of dispersibility in resins upon kneading. For example, it has been required to provide such hydrotalcite-type particles having a plate surface diameter of 0.2 to 1 xcexcm and a thickness of 0.05 to 0.2 xcexcm. However, the hydrotalcite-type particles obtained by the coprecipitation reaction have a plate surface diameter as small as 0.05 to 0.25 xcexcm and a thickness of 0.05 to 0.2 xcexcm. Therefore, in order to obtain hydrotalcite-type particles having a large plate surface diameter, it is necessary to use specific reaction conditions such as hydrothermal synthesis or the like.
Further, in the case where the hydrotalcite-type particles are used as a stabilizer for chlorine-containing resins or rubbers such as vinyl chloride resins or the like, it is required that the particles are not only sufficiently dispersed in the resins or rubbers to obtain resin or rubber products having an excellent heat resistance, but also have a higher chlorine ion-capturing ability.
In addition, as non- or low-toxic lead-free stabilizers for chlorine-containing resins, carboxylate, for example, zinc stearate are used as an essential component. However, when such carboxylates added as a stabilizer captures a chlorine gas released from the chlorine-containing resins, zinc chloride is produced by the reaction therebetween. As is known to those skilled in the arts, the thus produced zinc chloride acts as a decomposition catalyst for the chlorine-containing resins, so that carbon-carbon bonds constituting a skeleton of the resins are broken continuously, resulting in undesirable carbonization of the resins (so-called zinc burning). Accordingly, when used as a stabilizer for the chlorine-containing resins, the hydrotalcite-type particles are required to have a high ability of preventing the zinc burning.
At the present time, it has been strongly demanded to provide Mgxe2x80x94Al-based hydrotalcite-type particles which have a large plate surface diameter, an adequate thickness and a high zinc burning-preventing ability, and is suitable as a stabilizer for chlorine-containing resins. However, such Mgxe2x80x94Al-based hydrotalcite-type particles capable of satisfying the above properties have not been obtained yet.
Specifically, in the conventional coprecipitation method, it is not possible to obtain hydrotalcite-type particles having a large plate surface diameter. Under specific reaction conditions such as hydrothermal synthesis using the autoclave, it is possible to produce hydrotalcite-type particles having a large plate surface diameter. However, when the thus produced hydrotalcite-type particles are use as a stabilizer of chlorine-containing resins, the obtained chlorine-containing resin composition cannot show a sufficient heat resistance.
In particular, the chlorine-containing resin Compositions used as gasket materials are required to have an excellent heat stability and an adequate hardness. Especially, in the case of white or light-colored gasket materials, it is necessary that these materials are free from discoloration due to heat upon processing, i.e., have a good tinting property. As a stabilizer for such chlorine-containing resins satisfying these requirements, the hydrotalcite-type particles are required to exhibit a large plate surface diameter, an adequate thickness and a higher chlorine ion-capturing ability.
Further, the chlorine-containing resin compositions used as gasket materials contain a large amount of a plasticizer (for example, 50 to 100 parts by weight based on 100 parts by weight of the chlorine-containing resin) and, therefore, are soft materials, whereby the resin compositions tend to be foamed upon processing. In consequence, the foaming of the resin compositions should be avoided. In particular, due to the fact that the hydrotalcite-type particles are hydrous compounds, it is difficult to incorporate a large amount of the hydrotalcite-type particles into the resin compositions. Therefore, it has been strongly required to provide hydrotalcite-type particles capable of exhibiting an excellent effect even when added to resins in a small amount.
As a result of the present inventors"" earnest studies, it has been found that by mixing an aqueous anion-containing alkali solution, an aqueous magnesium salt solution and an aqueous aluminum salt solution with each other; after controlling a pH value of the solution to a specific range, aging the mixed solution at a specific temperature, thereby producing primary particles; then adding specific amounts of an aqueous magnesium salt solution and an aqueous aluminum salt solution to an aqueous suspension containing the primary particles; aging the aqueous suspension at a specific temperature and a specific pH value, thereby obtaining core particles; and forming on the surfaces of the obtained Mgxe2x80x94Al-based hydrotalcite-type core particles, a coating layer comprising at least one compound selected from the group consisting of higher fatty acids, organosilane compounds and rosins, the thus obtained Mgxe2x80x94Al-based hydrotalcite-type particles can exhibit a large plate surface diameter, an adequate thickness and a high zinc burning-preventing ability, and is suitable as a stabilizer for chlorine-containing resins. The present invention has been attained on the basis of this finding.
It is an object of the present invention to provide Mgxe2x80x94Al-based hydrotalcite-type particles which have a large plate surface diameter, an adequate thickness and a high zinc burning-preventing ability, and are suitable as a stabilizer for chlorine-containing resins.
It is another object of the present invention to provide a process for producing Mgxe2x80x94Al-based hydrotalcite-type particles which have a large plate surface diameter, an adequate thickness and a high zinc burning-preventing ability, and are suitable as a stabilizer for chlorine-containing resins.
It is a further object of the present invention to provide a chlorine-containing resin stabilizer capable of imparting an excellent stability and heat resistance to the resins.
To accomplish the aim, in a first aspect of the present invention, there are provided Mgxe2x80x94Al-based hydrotalcite-type particles, comprising:
(a) Mgxe2x80x94Al-based hydrotalcite-type core particles represented by the composition formula:
Mg1-x.Alx.(OH)2.Anxe2x88x92p.mH2O
xe2x80x83wherein 0.2xe2x89xa6xxe2x89xa60.6; p=x/n; A is an n-valent anion; and
m is more than 0 and not more than 1, and
(b) a coating layer formed on each Mgxe2x80x94Al-based hydrotalcite-type core particle, which comprises at least one compound selected from the group consisting of higher fatty acids, organosilane compounds and rosins; and
xe2x80x83having:
(c) a plate surface diameter of from 0.3 to less than 1.0 xcexcm,
(d) a thickness of from 0.02 to 0.08 xcexcm,
(e) a heat-resisting time of not less than 160 minutes when measured by the following methods (1) to (2):
(1) after the hydrotalcite-type particles together with additives are mixed in a vinyl chloride resin at the following mixing ratio, 50 g of the obtained mixture is kneaded at 155xc2x0 C. for 3 minutes using hot rolls whose gap is set to 0.75 mm, thereby obtaining a kneaded sheet.
Composition of Mixture:
(2) The obtained kneaded sheet is subjected to heat stability test according to JIS K 6723 to measure the heat-resisting time.
In a second aspect of the present invention, there is provided a stabilizer for chlorine-containing resins, comprising the Mgxe2x80x94Al-based hydrotalcite-type particles, comprising:
(a) Mgxe2x80x94Al-based hydrotalcite-type core particles represented by the composition formula:
Mg1-x.Alx.(OH)2.An+p.mH2O
xe2x80x83wherein 0.2xe2x89xa6xxe2x89xa60.6; p=x/n; A is an n-valent anion; and
m is more than 0 and not more than 1, and
(b) a coating layer formed on each Mgxe2x80x94Al-based hydrotalcite-type core particle, which comprises at least one compound selected from the group consisting of higher fatty acids, organosilane compounds and rosins; and
xe2x80x83having:
(c) a plate surface diameter of from 0.3 to less than 1.0 xcexcm,
(d) a thickness of from 0.02 to 0.08 xcexcm,
(e) a heat-resisting time of not less than 160 minutes when measured by the following methods (1) to (2):
(1) after the hydrotalcite-type particles together with additives are mixed in a vinyl chloride resin at the following mixing ratio, 50 g of the obtained mixture is kneaded at 155xc2x0 C. for 3 minutes using hot rolls whose gap is set to 0.75 mm, thereby obtaining a kneaded sheet.
Composition of Mixture:
(2) The obtained kneaded sheet is subjected to heat stability test according to JIS K 6723 to measure the heat-resisting time.
In a third aspect of the present invention, there is provided a process for producing the Mgxe2x80x94Al-based hydrotalcite-type particles as defined in the first aspect, comprising:
mixing an aqueous anion-containing alkali solution, an aqueous magnesium salt solution and an aqueous aluminum salt solution with each other;
after controlling the pH value of the solution to 10 to 14, aging the obtained mixed solution at a temperature of 80 to 105xc2x0 C. to produce primary particles of the Mgxe2x80x94Al-based hydrotalcite-type core particles;
adding an aqueous magnesium salt solution and an aqueous aluminum salt solution respectively containing magnesium and aluminum in such amounts that the molar ratio of a sum of Mg and Al added in this step to the sum of Mg and Al previously added upon formation of the primary particles is not more than 0.35:1, to the obtained aqueous suspension containing the primary particles;
aging the aqueous suspension at a temperature of 60 to 105xc2x0 C. while controlling the pH value thereof to 10 to 14: and
forming a coating layer comprising at least one compound selected from the group consisting of higher fatty acids, organosilane compounds and rosins, on the surfaces of the obtained Mgxe2x80x94Al-based hydrotalcite-type core particles.